1. Field of the Invention
This invention relates in general to submersible pumps for oil and water wells, and in particular to the pressure compensating system for these types of pumps.
2. Description of the Prior Art
Submersible pumps of the type concerned herein normally have a centrifugal pump that is driven by a submersible electric motor located below the pump. The shaft of the electric motor extends upwardly through a seal section for engaging the centrifugal pump. The seals are located in the seal section for sealing well fluid from the lubricating oil contained in the electric motor. Also, normally an elastomeric pressure compensator or a diaphragm is located in the seal section, with the drive shaft extending through a hole in the center of the compensator. A port exposes the exterior of the compensator to well fluid. The compensator is movable for reducing pressure differential between the well fluid and the lubricant in the motor.
A disadvantage of having the pressure compensator in the seal section is that if it leaks, the well fluid, being often mostly water, would migrate down into the motor, possibly causing damage. Also, having the pressure compensator in the seal section requires that it have a passage through its center for the drive shaft to pass.
Pressure compensators have been located below the motor in the prior art, such as shown in U.S. Pat. No. 4,436,488, Mar. 13. 1984 to Witten. If located below the motor, any leakage of well fluid would not likely contaminate the motor, since the well fluid would not migrate upward. However, problems are encountered in installing such an assembly in the well. The weight of the lubricant in the motor while at the surface would tend to move the bellows to a position of maximum lubricant volume. If the assembly is completely filled with lubricant, then when the motor starts to operate, the lubricant will not have any room to expand due to the rise in temperature. This would cause lubricant to be expelled across the seals, possibly causing damage to the seals.
Also, even if the bellows is initially in an intermediate position, leaving room for expansion, another problem might occur. If the assembly is suddenly stopped while lowering into the well, a downrush of lubricant could suddenly move the bellows from an intermediate position to a position of maximum lubricant volume, drawing a vacuum at the top of the seal section, possibly causing well fluid to be drawn in across the seals. In the above-mentioned patent, a pressure equalizing tube is taught as a solution in one embodiment, and loading the bellows with a counterbalancing fluid is suggested in another embodiment.